Hi
You may find it helpful to look in Answers, find a question section selecting the topic of Seasickness, as there is substantial advice available regarding preventative measures and remedies. With regard to Ship, Position and Technique, normally the deeper a ships keel, the less chance of seasickness, so a bigger ship would probably be more suitable, amidships is always best. A cabin in the middle of the ship, which is not too near the aft or back, or forward or front of the ship on a deck that is neither high nor low, would be the most appropriate. An outside cabin, which provides a view of the horizon, would be beneficial.
There are many preventative measures and remedies available, although the former is preferable to the latter, for example, medications, patches, sea bands, and even good old-fashioned Ginger; some pre departure research will definitely reduce the probability of problems on-board.
Hope this helps
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First of all I would suggest a fly/cruise avoiding a UK departure down the English Channel/Biscay route. A cruise starting from a Med or Caribbean type port where during the summer months movement on the newer larger vessels is imperceivable. The weather at this time of year is at it's worse in most cruising areas & that's why the subject of sea sickness gets over hyped, but if it was such a serious consideration do you honestly think cruising would have reached the popularity that it has. I have cruised with many first timers over the years & most have never known when we've set sail & any fears of sickness have been quickly dismissed.....Wilba.

Your parents might like to consider Norway - you rarely get waves in fjords. The North Sea has always been kind to us. The weather can be beautiful in the summer months and 24 hour daylight is a bonus. Bigger ships can be more stable in rough seas, but if you are not expecting rough seas, smaller ships tend to be more friendly, can berth in smaller ports and can get closer in than huge ships. Assuming that your parents are in their 70s, they may want to consider how many (if any) ports require tendering in, which can involve long queues. 'Family ships' may not suit them so an adult only ship might be a good idea. There are also 'themed' cruises - classical music, bridge, gardens etc. which may appeal.
Stugeron is a good remedy for seasickness and ginger tablets, available from health food shops, have been shown to be effective. If your father is really set against a cruise have they considered a rail trip e.g. Orient Express?

We love the Caribbean and avoid July to October which can have problems with hurricanes, but go usually Jan to May when the weather is still great temps 70/75 but in Jan/Feb not quite so great to sunbathe on the ships deck as the breeze can sometimes make it feel chillier due to the speed of the ship, but never noticed much in April and May and certainly on most days the sea was like a mill pond. Someone below commented on Norway and we have had two bad trips one with a force 12 gale in the North sea, we are good sailors and loved it but not everyone would like that sort of trip and again the Bay of Biscay we have so far been lucky and on 4 out of 4 trips not had any problem but later in the year will be our 5th trip so hopefully our luck will hold, hope your parents enjoy their annversary where ever they decide to go.